Disposable pattern composition for making same and method of investment casting

ABSTRACT

A disposable pattern material for investment casting processes incorporating cyanuric acid and a thermoplastic material; and an investment casting process utilizing a disposable pattern material incorporating cyanuric acid.

[ 11] 3,754,943 Aug. 28, 1973 Solomon DISPOSABLE PATTERN, COMPOSITION [56] References Cited FOR MAKING SAME AND METHOD OF UNITED STATES PAT INVESTMENT CASTING 3,263,286 8/1966 Watts etal l06/38.8 Inventor; Paul Solomon, Park 3225,42; aofiman etlal 106/388 7,9 /l9 er es eta. 106/ 8.8 [73] Assignee: Yates Manufacturing Co., Chicago, 8

Primary Examiner-Lorenzo B. Hayes [22] Filed: Sept. 30, 1971 Attorney- Max Dressler, Marshall W. Sutker et a].

2! A l. N 185, 74 l 1 PP O 3 57 ABSTRACT A disposable pattern material for investment casting [52] Cl 3 processes incorporating cyanuric acid and a thermo- 1 Int Cl Q plastic material; and an investment casting process uti- I a. a I v n s I s o e u u I a a l v a s I o u 1 I a [58] Field of Search 106/388, 38.25, uric acid 106/10,.270, 272, 230; ll7/l68; 260/285 R 8 Claims, No Drawings DISPOSABLE PATTERN, COMPOSITION FOR MAKING SAME AND METHOD OF INVESTMENT CASTING This invention relates to improved pattern materials, to improved disposable patterns especially made for use in investment casting processes, sometimes also known as lost wax processes, and to an improved investment casting process.

Investment casting processes have been used for centuries. Materials for making disposable patterns to be used in such processes are formulated for a number of properties, including important properties such as dimensional reproduceability and highly accurate surface finish in the molded disposable pattern. Because such properties are critically important to many products made by lost wax processes, continuing efforts are always underway to improve those properties of pattern materials, among others.

Virtually all of the properties of an investment casting depend upon the quality of the disposable pattern. These in turn depend upon the characteristics of the pattern forming materials from which disposable patterns are molded.

Disposable thermoplastic patterns are usually formed by heating and melting a thermoplastic pattern forming composition, introducing the molten composition into a mold, and then cooling the composition until it solidifies to form a disposable pattern. Thereafter the disposable thermoplastic pattern is removed from the mold, is assembled, if necessary, with other patterns, and is then encased in a mold forming material, usually a ceramic material, in accordance with one of a variety of known methods, thereby to form a shell or cast about the disposable pattern. The disposable pattern is then removed, as by melting or vaporizing the pattern material, so that it leaves the shell or cast. Thereafter the shell or mold is ready for one-time use for forming an investment cast part. A text describing known procedures used in lost wax processes is entitled Investment Casting, H. T. Bidwell, Machinery Publishing Co., Ltd., England, 1969.

It is apparent that the surface characteristics of the disposable pattern and of the ceramic shell are transferred, so to speak, to the final casting. It is also apparent that the pattern material and any residue therefrom will affect the surface and metallurgical characteristics of a casting. Similarly, it is clear that variations in expansion and contraction of compositions from which disposable patterns are formed will result in shells or casts of varying dimensions, which will then produce inconsistent castings. It is for such reasons that the properties of pattern materials are critical to the investment caster.

Many thermoplastic pattern materials have been used and have been suggested for use in the past. As the name lost wax" process implies, true waxes, such as natural waxes, as beeswax and the like, were originally used as thermoplastic pattern materials. As other pattern materials were sought to improve the properties of disposable patterns, other natural thermoplastic materials, such as gum damar, gum rosin, esparto waxes, and the like, mineral waxes, such as those extracted from soft coal, and the like, and petroleum waxes were adopted for use. Subsequently, modified waxes, such as microcrystalline waxes, were developed for use and used in lost wax processes. More recently, synthetic thermoplastic materials, such as polystyrene, have been used as pattern materials or as thermoplastic pattern forming composition modifiers as a result of the continuing efforts of researchers to improve upon and develop new thermoplastic materials. Those efforts have also resulted in the use by some investment casters of materials other than thermoplastic pattern materials, such as mixtures of metallic salts, mercury, among others.

Efforts have also been made to increase the dimensional accuracy and stability of thermoplastic pattern forming compositions by the addition of powdered solid filler materials. Polystyrene powder and urea powder have been so used, and have been added in minor quantities to thermoplastic pattem-forming compositions. Organic acids, such as fumaric, adipic, and isophthalic acids, have also sometimes been used as solid fillers, usually in amounts of up to about 40 percent by volume of the thermoplastic pattern forming composition, and in a particle size generally in the range of about 175 to 250 mesh.

In accordance with my invention, I have discovered that a thermoplastic pattern composition comprising an organic thermoplastic pattern material, and from about 5 percent to about percent and preferably from about 20 to about 70 percent of finely divided cyanuric acid by volume of the thermoplastic pattern forming composition produces disposable thermoplastic patterns for use in investment casting processes which have superior dimensional reproduceability and highly accurate surface finishes, in addition to providing disposable patterns which have excellent characteristics in other important regards as well. Cyanuric acid in this amount in pattern forming compositions of waxes, such as true waxes, and resins used in lieu of waxes, produces a thermoplastic disposable pattern which has a number of advantages, and which has improved characteristics, as compared to those disposable thermoplastic patterns currently in vogue. The terms organic thermoplastic material or thermoplastic material are sometimes used in this application to include waxes, and resins frequently referred to as waxes, such as polystyrene, and other presently useful or potentially useful waxes and natural and synthetic thermoplastic resins, such as rosin and polystyrene, which are usable as the thermoplastic portion of a thermoplastic pattern forming composition incorporating cyanuric acid.

When disposable patterns are formed of organic thermoplastic materials and finely divided cyanuric acid, upon cooling, they shrink and contract considerably less than the thermoplastic material itself would shrink. Other improved properties result from the use of cyanuric acid in thermoplastic pattern forming compositions. For example, because cyanuric acid is dimensionally and physically stable up to 330C., a temperature considerably higher than the melting point of thermoplastic materials currently used to make disposable patterns, higher drying temperatures can be used during the application of ceramic coats to the thermoplastic patterns in the ceramic shell lost wax process. That speeds quite considerably the shell making portion of such investment casting processes. Additionally, because cyanuric acid decomposes without melting at about 330C., when flash de-waxing procedures are used in an investment casting process, substantially no thermal expansion is encountered from the cyanuric acid portion of the disposable pattern, minimizing the potential of distortion, breakage and cracking of the ceramic casts or shells. The decomposition of cyanuric acid without melting also minimizes pitting of the ceramic casting surface. That is because currently used meltable waxes and fillers tend to boil against the ceramic casting surface and erode the interface.

A number of other problems are alleviated or avoided when thermoplastic pattern forming compositions in accordance with this invention are used. Other advantages derived from the practice of this invention and of disposable -patterns made in accordance with this invention will become apparent from the following description and examples.

The composition of this invention comprises a thermoplastic pattern material and cyanuric acid (2, 4,

6-trihydroxy-s-triazine) in an amount. of from about to about 70 percent by volume of the total thermoplastic pattern forming composition. For high quality castings, the particle size should not exceed 100 mesh. Cyanuric acid does not appreciably expand or shrink in a range from ambient room temperature to a temperature of 330C., at which upper temperature it decomposes without first melting. Cyanuric acid is the only material of which I am presently aware which has those properties and which, in the amounts stated, produces two-phase disposable thermoplastic patterns for use in investment casting processes, which patterns have a combination of the highly desirable characteristics described herein.

Cyanuric acid exists both in the enol and keto tautomeric forms. Therefore it may be called either isocyanuric or cyanuric acid, and it is to be understood that the term cyanuric acid is used herein to encompass both forms of acid. It is an infusible crystalline solid. Its molecular weight is 129.08 (C 1-1 O N and its true density is 1.73 gm/cc (at 25C.). As stated above, it decomposes without melting at 330C. In pattern forming compositions cyanuric acid is inert, hence it is not subject to shrinkage upon cooling, as are the thermoplastic portions of thermoplastic pattern forming compositions.

Compositions according to this invention have been formed into solid disposable pattern and have been tested.

EXAMPLE 1 1 A thermoplastic pattern material was made by melting and mixing ingredients as follows until they were homogeneously dispersed.

Parts Ingredients by Volume Terpene polymer (115C. m.p.) 55 S nthetic paraffinic mineral wax 5 00C. mg.) Paraffin (l 8 to 140F. mp.) 20 Natural camauba wax to Microcrystalline wax (175-l80C. m.p.) l 0 EXAMPLE 2 A mixture of 60 parts by volume of finely divided cyanuric acid and 40 parts by volume of the mixture of Example 1 was formulated. The cyanuric acid was blended into the homogeneous melt of Example 1, but retained its particulate identity. The result was a two phase system with the cyanuric acid particles uniformly distributed throughout. The finely divided cyanuric acid used had a screening analysis as follows:

mesh 94.3% mesh 83.6% passes 200 mesh 45.2% passes 325 mesh 23.9% passes The ball and ring softening points of solid body test patterns cast from the compositions of Examples 1 and 2 were about the same, i.e., F. and 168F., respectively. However, the volumetric expansions of the test patterns were markedly different, and as will be apparent, the volumetric expansion characteristics of the test pattern of Example 2 was very markedly improved.

Volumetric Expansion The accuracy of that test method is considered to be about $0.2 percent. The test results demonstrate a very substantial reduction in thermal expansion, a highly desirable property for disposable thermoplastic patterns.

It was also determined that the pattern of Example 2 was much less likely to break a ceramic cast when the pattern was eliminated. Additionally, the disposable pattern of Example 2 produced less residual ash and less erosion and wetting of the ceramic surface, thereby utlimately producing a casting having a more accurate surface finish. That is especially important in pattern removal processes where temperatures of as much as l,600 to 2,000F. are reached, thereby to eliminate wax and carbon, and where the thermoplastic ingredients of disposable patterns melt and boil against the ce-' ramic mold surface. Because'the cyanuric acid portion of the composition does not melt, ceramic surface erosion is minimized when cyanuric acid is present in a disposable thermoplastic pattern.

Other compositions embodying the principles of this invention have been made. Disposable thermoplastic patterns formed from those compositions have demonstrated a reduction in ash content and a reduction in the agitation caused by boiling and burning against the ceramic mold during de-waxing, and have also demonstrated improved reproduceability and surface finish characteristics.

Thermoplastic pattern forming compositions were blended as indicated in, Examples 1 and 2, of the following ingredients.

The ash content of Example 3 was 0.013 percent of the original volume.

EXAMPLE 4 Parts by Volume In em Ma i; of Example 3 Powdered cyanuric acid EXAMPLE 5 Ingredient Mixture of Example 3 Powdered cyanuric acid Parts by Volume The ash content of Example 5 was found to have been reduced to 0.004 percent of the original volume.

In Examples 4 and 5, the surface finishes of castings cast from a disposable pattern of the compositions indicated were found to be substantially improved as compared to the surface finish of castings cast from a disposable pattern made from the composition of Example 3.

As far as I am aware, efforts to use prior art fillers, such as those described above, with higher temperature waxes to form disposable patterns have failed, largely because such prior art fillers themselves melt, decompose, or otherwise fail to function properly at elevated temperatures. When high temperature thennoplastic pattern materials, such as waxes, can be used to form patterns, they can speed the ceramic mold or cast forming step of an investment casting process because higher temperatures can be used to dry the ceramic shell or cast, thus decreasing the drying time. Higher melting thermoplastic materials also frequently exhibit greater dimensional stability, both at higher and lower temperatures, than do those lower temperature disposable pattern materials now in use, making the use of high temperature thermoplastic materials desirable. When cyanuric acid is used, higher temperature waxes and other thermoplastic materials may also be used when desired.

EXAMPLE 6 A high temperature thermoplastic pattern material was compounded, as stated above, from the following Ingredient Parts by Volume Acrawax C (MW-143C. m.p. Glyco Products) 50 Powdered cyanuric acid This produced a two-phase thermoplastic pattern composition which was quite stable and which did not melt below about 140C. Its volumetric expansion was considerably lower than those of Examples 1 and .2, as shown by the following table:

Volumetric Expansion (ASTM D401; Method B) unacceptable for the production of high quality investment castings. By using a substantial fraction of finely divided cyanuric acid, a significant enough reduction in the expansion of polystyrene pattern material may be realized to make polystyrene powders useful where they were not before. For example, a thermoplastic pattern forming composition comprising 50 parts by volume of a methyl styrene copolymer (such as Resin 24X Amoco Chemical Company) having a melting point of 238F., and 50 parts by volume of cyanuric acid is such a composition.

Other formulations, incorporating thermoplastic pattern materials not heretofore generally considered useful in lost wax processes which may now be advantageously considered for possible commercial use are the following. They may be blended to form two-phase sys tems in the manner indicated above.

EXAMPLE 7 Ingredient Parts by Volume Crystalline Polyolefin resin, 270F. m.p. 5O Powdered cyanuric acid it EXAMPLE 8 In edient Parts by Volume E ylene isobutyl acrylate copolymer, 2501-25F. m.p.

Powdered cyanuric acid las EXAMPLE 9 Ingredient Parts by Volume Ethylene ethyl acrylate copolymer, 180F. m.p. Powdered cyanuric acid 2 5 I EXAMPLE l0 Ingredient Parts by Volume Microcrystalline wax, ll95"F. m.p. 50 Powdered cyanuric acid 5 0 Other thermoplastic pattern materials which appear to have potential and which produce improved thermoplastic patterns because of the presence of cyanuric acid are the following:

It is clear that this invention provides improved disposable thermoplastic pattern forming materials and improved disposable patterns, i.e., solid bodies of any predetermined shape, and consisting of a two-phase system in which the cyanuric acid particles are uniformly distributed. It provides an improved method of investment casting in which disposable patterns are formed of compositions of this invention, and are then invested, as in ceramic or a refractory or the like, to form an improved cast.

Other thermoplastic pattern forming compositions will become apparent to those skilled in the art from the foregoing. It is my intention that those equivalent compositions shall also be considered as being within the scope of the appended claims.

I claim:

I. In a two-phase thermoplastic pattern forming composition comprising a thermoplastic pattern material and a finely divided solid filler material, the improvement comprising, as said filler material, cyanuric acid in an amount of from about to about 70 percent by volume of the pattern forming composition.

'2. A thermoplastic pattern forming composition in accordance with claim 1 in which said pattern material is a true wax.

3. A thermoplastic pattern forming composition in accordance with claim 1 in which said thermoplastic pattern material is a polystyrene resin.

4. A thermoplastic pattern forming composition in accordance with claim 1 in which said cyanuric acid is present in an amount of from about 20 to percent, by volume.

5. In a disposable thermoplastic pattern for investment casting comprising a solid body of a predetermined shape, said body comprising a two-phase mixture of a thermoplastic pattern material and a finely divided solid filler material, the improvement comprising, as said filler material, about 5 to about 70 percent by volume of said body of finely divided cyanuric acid.

6. A disposable pattern in accordance with claim 5, wherein said pattern material is a true wax.

7. A disposable pattern in accordance with claim 5, wherein said pattern material is a polystyrene resin.

8. In a method of investment casting comprising forming in a mold a two-phase disposable pattern of a thermoplastic pattern forming composition comprising a thermoplastic pattern material and a finely divided solid filler material, and then investing said pattern to form a cast, the improvement comprising utilizing as said filler material finely divided cyanuric acid in an amount of from about 5 to about 70 percent by volume of said composition.

UNITED STATES PATENT CFFICE CERTIFICATE OF CORRECTION Patent No. 3,75 +,9H3 Dated Augu t 2 ,.1'973 Invcntor(s) Paul Solomon It is certified that error appears in the. above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 'line &4, "pattern" should be patterns line 52, "Ingredients" should be Ingredient --5 Column line 1, after "94.3%" insert passes (Claim 1) Column 7, line 21, after the numeral "5 insert percent -3 (Claim I) Column 8, line 4, after the numeral "20" insert percent -3 (Claim 5) Column 8, line 11, after the numeral "5" insert percent --5 (Claim 8) Column 8, line 24, after the numeral 5" 7 insert percent Signed and sealed this 27th day of November 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. RENE D. TEGTMEYER Attest n g Officer Acting Commissioner of Patents 

2. A thermoplastic pattern forming composition in accordance with claim 1 in which said pattern material is a true wax.
 3. A thermoplastic pattern forming composition in accordance with claim 1 in which said thermoplastic pattern material is a polystyrene resin.
 4. A thermoplastic pattern forming composition in accordance with claim 1 in which said cyanuric acid is present in an amount of from about 20 to 70 percent, by volume.
 5. In a disposable thermoplastic pattern for investment casting comprising a solid body of a predetermined shape, said body comprising a two-phase mixture of a thermoplastic pattern material and a finely divided solid filler material, the improvement comprising, as said filler material, about 5 to about 70 percent by volume of said body of finely divided cyanuric acid.
 6. A disposable pattern in accordance with claim 5, wherein said pattern material is a true wax.
 7. A disposable pattern in accordance with claim 5, wherein said pattern material is a polystyrene resin.
 8. In a method of investment casting comprising forming in a mold a two-phase disposable pattern of a thermoplastic pattern forming composition comprising a thermoplastic pattern material and a finely divided solid filler material, and then investing said pattern to form a cast, the improvement comprising utilizing as said filler material finely divided cyanuric acid in an amount of from about 5 to about 70 percent by volume of said composition. 